1. Field of the Invention
The present invention relates to low-noise optical probes which may be used to sense optical energy passed through or reflected from a medium to determine the characteristics of the medium.
2. Description of the Related Art
The physical characteristics of a given medium may often be determined by transmitting electromagnetic or acoustic energy through, or reflected energy from, portions of the medium. For example, in the context of medical diagnosis, light or sound energy may be directed onto a portion of a patient""s body, and the fraction of that energy transmitted through (or reflected by) the patient""s body measured to determine information about the various physical attributes of the patient. This type of non-invasive measurement is both more comfortable for and less deleterious to the patient than invasive techniques, and can generally be performed more quickly.
Non-invasive physiological monitoring of bodily function is often required. For example, during surgery, blood oxygen saturation (oximetry) is often continuously monitored. Measurements such as these are often performed with non-invasive techniques where assessments are made by measuring the ratio of incident to transmitted (or reflected) light through an accessible part of the body such as a finger or an earlobe. A typical transmissive non-invasive monitoring device includes a light source such as a light-emitting diode (LED) placed on one side of the body part, while a photodetector is placed on an opposite side of the body part. Light energy generated by the LED is transmitted through the tissue, blood, and other portions of the body part, and detected by the photodetector on the other side. Alternatively, in a reflective device, the detector is placed on the same side of the body part as the light source, and the amount of light energy reflected by the body part measured.
The transmission of optical energy passing through the body is strongly dependent on the thickness of the material through which the light passes (the optical path length). Many portions of a patient""s body are typically soft and compressible. For example, a finger comprises a number of components including skin, muscle, tissue, bone, and blood. Although the bone is relatively incompressible, the tissue, muscle, and skin are easily compressible or deformed with pressure applied to the finger, as often occurs when the finger is bent. Thus, if optical energy is made incident on a patient""s finger, and the patient moves in a manner which distorts or compresses the finger, the optical properties, including optical path length, may change. Since a patient generally moves in an erratic fashion, the compression of the finger is erratic and unpredictable. This causes the change in optical path length to be erratic, making the absorption of incident light energy erratic, and resulting in a measured signal which can be difficult to interpret. Similarly, movement of the patient during a reflective measurement can dramatically affect the quality of the signal obtained therefrom.
In addition to the typical problem of patient movement, the presence of unwanted ambient and/or reflected light energy interferes with the measurement of the intensity of the light transmitted through or reflected by the body part. Optical transmission/reflection systems as described above utilize a light energy detector which measures, inter alia, the intensity of light transmitted to or reflected from the body part being analyzed. Since ambient light incident on the detector affects the intensity measurement, noise or error is introduced into the measured signal by such ambient light. Similarly, light generated by the light source within the measuring device (typically, an LED) which is not transmitted through or reflected by the body part under examination will also result in signal error if such light is received by the detector. These xe2x80x9csecondaryxe2x80x9d reflections arise when light emitted by the light source is reflected by structures within the optical probe onto the detector. Accordingly, to increase the accuracy of the measurement process, both ambient light and xe2x80x9csecondaryxe2x80x9d reflections from the light source should be mitigated.
FIG. 1a illustrates an ideal signal waveform obtained from an optical probe system. FIG. 1b illustrates an actual spectra obtained from a typical optical probes not corrected for the effects of patient motion or ambient/reflected light. Note the significant increase in noise (and resulting loss of signal clarity) in FIG. 1b due to these effects.
Prior optical probes have successfully addressed the issue of ease of use and patient motion during measurement. See, for example, U.S. Pat. No. 5,638,818 entitled xe2x80x9cLow Noise Optical Probe,xe2x80x9d assigned to the Applicant herein, which discloses a system utilizing a chamber which isolates that portion of the patient""s tissue under examination from compression or movement by the patient. The device is attached to the finger of a patient, thereby readily and accurately positioning the tissue of the patient""s finger over the chamber.
However, attempts at limiting the effects of ambient and xe2x80x9csecondaryxe2x80x9d reflected light have been less successful, not due to their ineffectiveness, but rather due to their obtrusiveness and relative complexity of use. A need exists, especially in the health care context, for a simple, fast, unobtrusive, and largely error-free means of non-invasive measurement of a patient""s physical parameters. Especially critical is the attribute that such means be easily adapted to a variety of different patient types and characteristics with little or no adjustment, as is the device disclosed in the aforementioned patent. Prior art methods of mitigating ambient and reflected light interference have involved coverings or shrouds which substantially envelop the optical probe and tissue, thereby requiring substantial sizing and adjustment of the covering for each different patient being measured. Another disadvantage of such methods is that the placement of the patient""s appendage (such as a finger) in relation to the light source and detector can not be reliably verified by the person administering the measurement unless the probe is first placed on the appendage, and the covering installed thereafter, or alternatively, unless the patient is queried. This necessitates additional time and effort on the part of the patient and the person making the measurement.
Another factor relating to the efficacy of an optical probe is force distribution on the body part or tissue material being measured. Specifically, if force is distributed on the tissue material being measured unevenly or disproportionately, varying degrees of compression of the tissue may result, thereby producing a broader range of optical path lengths in the region of the light source and detector. Furthermore, if the force that the probe exerts on the tissue material is highly localized, the ability of the patient to move the tissue material with respect to the source/detector is enhanced, thereby leading to potentially increased noise levels within the signal generated by the probe.
Yet another consideration relating to non-invasive optical probe measurement involves cost. In recent times, the demand has increased significantly for both disposable and reusable optical probes which are suitably constructed to provide accurate, low-noise measurements. The aforementioned prior art methods of attenuating ambient and reflected light employing coverings or shrouds carry with them a significant cost, especially if the probe (or components thereof) must be replaced on a frequent basis. Therefore, in many applications, it would be useful to have a low-cost reusable optical probe capable of attenuating ambient and reflected light, with only the degradable components being easily and cost-effectively replaced as required, without necessitating the replacement of the entire probe. Similarly, it would be useful to have a disposable probe capable of attenuating ambient and reflected light, which could be routinely replaced in its entirety a cost-effective manner.
Finally, existing optical probes do not include an easy to use and reliable means for determining when to replace the probe. At present, the probe operator or health care provider must keep a record or log of the date of installation of a given probe, and replace it at a given periodicity or simply replace the probe when it seems worn out. This approach is problematic, however, not only from the standpoint of additional time and effort consumed in maintaining the record, but more significantly from the perspective that the measurement of installed time is not necessarily representative of the wear on the probe. For example, two probes installed on the same date may experience significantly different levels of wear, depending on the level of use. Alternatively, the operator could keep a log of usage, but this is too burdensome and time consuming.
Based on the foregoing, a need exists for an improved low-noise optical probe which (i) is simple in design and easy to use under a variety of different operating conditions; (ii) is capable of attenuating ambient and reflected light without necessitating probe adjustment or fitting to each different patient; (iii) is capable of alerting the operator when replacement is required; and (iv) is cost effective. Such an improved probe would also ideally shield against noise caused by electromagnetic interference (EMI).
The present invention satisfies the foregoing needs by providing an improved optical probe for use in non-invasive energy absorption or reflection measurements, as well as a method of using the same.
In a first aspect of the invention, an improved shielded optical probe assembly is disclosed which incorporates a light energy source and light energy detector embedded within a multi-part housing adapted to receive and clamp onto tissue material from the patient. When the probe is operating, light energy is directed from the light energy source through a first aperture formed within a first element of the housing and onto the tissue material of the patient, which is received within the probe. A portion of this light is transmitted through (or reflected from) the tissue material onto the detector via a second aperture. In this fashion, a light generated by the light source and transmitted through or reflected from the tissue material at a localized point is received by the detector. A light shield is fitted to the housing so as to partially surround the tissue material when it is received within the housing, thereby attenuating ambient light incident on the optical probe. In one embodiment, the light shield is made removable in order to facilitate its replacement after degradation and wear. Additionally, portions of the shield and housing are colored and/or coated such that light incident on these portions is absorbed or attenuated. The foregoing light attenuation features act to reduce the effects of noise induced within the detector (and associated processing circuitry) due to light energy not transmitted directly through or reflected from the tissue material from the light source. The probe is also optionally fitted with a diffraction grating and Faraday shield to mitigate the effects of unwanted optical modes and electromagnetic interference on probe accuracy.
In a second aspect of the invention, the foregoing optical probe includes a mechanism for equalizing the force applied to the tissue of the patient when the probe is clamped thereon. In one embodiment, a series of elongated apertures each receive hinge pins which are biased apart by springs wound around the axis of the pins. When the housing elements of the probe are grasped and compressed together by the user, the hinge pins are forced against one edge of the elongated apertures, thereby providing a fulcrum for opening the probe. After the probe is opened, and the patient""s finger inserted, the compressing force is removed, thereby allowing the housing elements to clamp onto the finger. As the compression force is removed, the spring bias allows the previously compressed ends of the housing elements apart, and urging the pins to the opposite edge of the elongated apertures, and xe2x80x9clevelingxe2x80x9d the housing elements into a more parallel orientation. This parallel orientation distributes force on the patient""s finger more evenly.
In a third aspect of the invention, a monitoring device is disclosed which is integrated with the optical probe circuitry in order to assist the operator in determining when to replace the probe. In one embodiment, the monitoring device is a counter which counts the number of electrical pulses generated by the detector circuitry, and correlates this number to the time of actual probe operation and percent of useful lifetime. A light emitting diode visible on the exterior of the probe is used to alert the operator to the need for probe replacement.
In another aspect of the invention, a method of measuring the amount of light transmitted or reflected by the tissue material of a patient using the aforementioned optical probe is disclosed. In one embodiment of the method, the tissue material is inserted into the shielded probe housing, and light generated by the light source of the probe is transmitted via the first aperture into the tissue material. Light energy transmitted (or reflected) by the tissue material is then detected by the detector via the second aperture, and a signal relating to the intensity of the detector generated. Ambient light incident on the probe, and light generated by the light source and scattered off components other than the tissue material, are attenuated or absorbed by the shield and absorptive coating(s) during detection and signal generation in order to reduce any noise component associated therewith. The operating time of the probe is also counted in order to monitor probe remaining lifetime.